In recent years, concerning illumination light sources for luminaires, more and more incandescent lamps and fluorescent tubes are replaced with light sources that consume less power and have longer life such as an LED (Light Emitting Diode). New illumination light sources such as an EL (Electro-Luminescence) and an OLED (Organic light-emitting diode) are developed. Since the luminance of these illumination light sources depends on a current value flowing thereto, when the illumination light sources are lit, a power-supply circuit that supplies a constant current is necessary. In order to adjust a direct-current power supply voltage to a rated voltage of an illumination light source, usually, step-down means is used. As step-down means having high current usage efficiency, a self-excitation DC-DC converter is proposed (see, for example, JP-A-2004-119078).
In an LED lighting device described in JP-A-2004-119078, an FET (Field-Effect Transistor), a resistor for current detection, a first inductor, and an LED circuit are connected to a direct-current power supply in series to form a loop-shape main current path. A voltage generated by resistance division of an output of the direct-current power supply is applied between a source and a gate of the FET. A voltage between both ends of the resistor for current detection is also applied between the source and the gate. A diode is connected between both ends of the first inductor and the LED circuit to form a loop-shape feedback circuit. Further, a second inductor magnetically coupled to the first inductor is provided such that an electromotive force of the second inductor is applied to the gate of the FET.
In such an LED lighting device, when a power-supply is turned on, potential generated by resistance division of a power-supply voltage is applied to the gate of the FET and the FET changes to an ON state. An electric current starts to flow to the main current path. When this electric current increases, an electromotive force is generated in the second inductor and the FET is kept on. Consequently, the LED circuit is lit and magnetic energy is accumulated in the first inductor. Thereafter, when the electric current flowing through the main current path reaches a predetermined amount, a voltage drop amount between both the ends of the resistor for current detection reaches a predetermined amount, gate potential with respect to the source potential of the FET falls to be lower than a threshold, and the FET changes to an OFF state. Consequently, the main current path is shut off. An electric current flows to the feedback circuit with the magnetic energy accumulated in the first inductor and lights the LED circuit. At this point, since this electric current decreases with time, an opposite electromotive force is generated in the second inductor and the FET is kept off. Thereafter, when the electric current decreases to zero, the direction of the electromotive force of the second inductor is reversed again and the FET changes to the ON state. According to the repetition of such operation, self-excitation DC-DC conversion is performed and a stepped-down DC voltage is supplied to the LED circuit.
However, in the LED lighting device in the past, the resistor for current detection is necessary. When the FET is on, an electric current always flows to the resistor for current detection. Therefore, a loss of electric power is large. If the resistor for current detection is not used, a heavy current is likely to flow during the start.
It is an object of the present invention to provide a switching power supply and a luminaire in which a loss of power is small and an overcurrent during the start is suppressed.